Method and device for rapid cutting of a workpiece from a brittle material

ABSTRACT

The method for rapid cutting of a workpiece made of brittle material along a predetermined cutting line of any desired shape includes generating laser beams, preferably with a CO or CO 2  laser; focusing the laser beams onto the cutting line to form focused laser beams on the cutting line; guiding the focused laser beams one behind the other along the cutting line without melting the brittle material; shaping the respective laser beams so that the respective beam cross-sections forming corresponding focal spots on a surface of the workpiece have predetermined shapes and intensity distributions; moving the workpiece and the laser beams relative to each other along the cutting line so that the focused laser beams induce a thermo-mechanical stress in the brittle material and blowing a fluid cooling medium, such as cold air or an air/water mixture, onto a heated cutting line section of the workpiece for subsequent cooling so as to increase the thermo-mechanical stress in the brittle material above its breaking strength.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and a device for the rapid cutting ofa workpiece made from brittle material, in particular made from glass,glass-ceramic or ceramic, by means of laser beams along a cutting lineof any desired contour. A preferred application is he rapid cutting offlat glass.

2. Description of the Related Art

Conventional methods for cutting glass are based on using a diamond or asmall cutting wheel first of all to produce a score in the glass, inorder for the glass then to be broken along the weakened line generatedin this way by the application of an external mechanical force. Adrawback of these methods is that the score causes particles (splinters)to be detached from the surface, and these particles can be deposited onthe glass, where they may, for example, cause scratches. Also, chips mayform at the cut edge, leading to an uneven glass edge. Furthermore, themicrocracks in the cut edge which form during the scoring operation leadto a reduced mechanical load-bearing capacity, i.e. to an increased riskof breaking.

One approach aimed at avoiding both splinters and chips and microcracksis to separate glass using thermally generated stress. In this case, aheat source which is directed onto the glass is moved at a fixedvelocity relative to the glass, and in this way generates a thermalstress which is so high that the glass forms cracks. The requiredproperty of the heat source of being able to position the thermal energylocally, i.e. with an accuracy of better than one millimeter, whichcorresponds to typical cutting accuracy, is satisfied by infraredradiators, special gas burners, and in particular lasers. On account oftheir good focussing properties, good controllability of the power andthe possibility of beam shaping and therefore intensity distribution,lasers have proven successful and achieved widespread use on glass.

This laser-beam cutting method, which induces a thermomechanical stressto above the breaking strength of the material by local heating by meansof the focussed laser beam in combination with external cooling, hasbeen disclosed by a number of documents.

The method which is known from WO 93/20015 uses a laser beam ofelliptical shape with a trailing cooling spot. This method achieves goodresults in straight-line scoring of nonmetallic plate material, but isunable to ensure high-quality and highly accurate scoring along a curvedcontour. Moreover, the known method does not achieve a very stablecutting profile at a high radiation density and high cutting speeds.

To optimize the heating conditions of the material along the cuttingline, according to WO 96/20062 the heating is effected by means of abundle of heating beams, in the cross section of which passing throughthe center of the bundle the density of the radiation power isdistributed so that it decreases from the periphery toward the center.An elliptical beam bundle is used, resulting in temperature distributionin the form of an elliptical ring.

The drawbacks of these known methods are avoided by the method describedin EP 0 872 303 A2, which provides a focal spot which has a U-shaped orV-shaped contour which opens out in the cutting direction and acharacteristic intensity distribution. This method has proven successfulin practice when carrying out straight cuts. It is possible to cutthrough even large workpiece thicknesses cleanly. When carrying outfree-form cuts, i.e. cuts with any desired contour, possibly including acurved contour, it is necessary to generate a curved U-shaped orV-shaped intensity distribution which is matched to the contour of thecutting line and for the contour to be tracked, including the subsequentcooling. This requires in particular coupling of the scanner devicewhich generates the focal spot with a path control unit, which entails anot inconsiderable control and adjustment outlay.

DE 44 11 037 C2 has disclosed a laser-beam cutting method for cuttinghollow glasses which operates with a stationary laser beam which issharply focussed to form a spot and generates a thermal stress zonearound the rotating hollow glass. Then, cooling is effected along thestress zone which has been introduced over the entire periphery of thehollow glass using a mist of atomized water which is blown out of anozzle, so that the hollow glass edge is severed when used inconjunction with a mechanically or thermally generated starting crack.

DE 43 05 107 A1 has disclosed a laser-beam cutting method in which thelaser beam is shaped in such a way that its beam cross section, on thesurface of the workpiece, is elongate in shape, in which method theratio of length and width of the impinging beam cross section can be setby means of a diaphragm in the laser-beam path.

Furthermore, methods for cutting glass by means of a plurality of laserbeams are known.

In the method described in DE-B 1 244 436, inter alia to produce shapededges, more than one laser beam is applied to the same cut during thelaser cutting, and the individual laser beams form different angles withthe glass surface. The corresponding U.S. Pat. No. 3,453,097 describes amethod for cutting glass by means of a plurality of laser beams guidedin coupled form onto the cutting line.

For a very wide range of reasons, the first laser-beam cutting methoddescribed has proven to be the superior method and has become widelyaccepted in practice. The invention is based on this method. The cuttingcapacity which can be achieved by the first method described and theusability of the method are governed in particular by the effectiveinduction of a thermomechanical stress along the cutting line in theworkpiece which is to be cut, the intensity distribution in the laserbeam and the type of cooling. The rapid and, at the same time, effectiveinduction of a thermomechanical stress along the cutting line which isrequired for cutting is only ensured to an insufficient extent in theknown methods. On account of the ineffective induction of athermomechanical stress, limits are imposed on the demand for everhigher cutting speeds.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and adevice for cutting a workpiece made from brittle material, in particularmade from glass, glass-ceramic or ceramic, by means of laser beams alonga predetermined cutting line of any desired contour, so that a highcutting accuracy and faithfulness to contours and rapid cutting arepossible without the formation of microcracks, chips or splinters.

This object is attained, according to the invention, by a method forrapid cutting of a workpiece made of brittle material along apredetermined cutting line of any desired shape by means of laser beams,which comprises:

a) generating the laser beams;

b) focusing the laser beams onto the cutting line to form focused laserbeams on the cutting line;

c) guiding the focused laser beams one behind the other along thecutting line without melting the brittle material;

c) shaping the respective laser beams so that the respective beamcross-sections forming corresponding focal spots on a surface of theworkpiece which is to be cut have predetermined shapes and intensitydistributions;

d) moving the workpiece and the laser beams relative to each other sothat the focal spots move along the cutting line and the focused laserbeams induce a thermo-mechanical stress in the brittle material; and

e) blowing a fluid cooling medium onto a heated cutting line section ofthe workpiece for subsequent cooling so as to increase thethermo-mechanical stress in the brittle material above a breakingstrength of the brittle material.

Clean cut edges are achieved, without any microcracks, chips orsplinters.

The method according tothe invention allows rapid and effective inducingof a thermomechanical stress along a cutting line of any desiredcontour. Surprisingly, it has been found that high laser powers can beintroduced into the workpiece to be cut without the workpiece melting.At the same time, it was possible to increase the cutting speed by up to100% compared to previous methods.

In this way, it is readily possible to achieve cutting speeds of up to200 m/min.

Particularly in order to introduce high laser powers into the workpiecewithout the workpiece being melted at the same time, it is advantageousfor the laser beams which are guided in coupled form, and in particularin parallel, to be guided onto the cutting line one behind the other. Inthis case, it is possible for the laser beams to be guided in quicksuccession one behind the other or for the laser beams to be guided ontothe cutting line in completely or partially superimposed form.

The respective laser beams are in this case preferably shaped in such amanner that the beam cross sections acting as focal spots on the surfaceof the workpiece to be cut in each case correspond to the same shape andthe same intensity distribution.

Furthermore, the laser beams are preferably guided onto the cutting lineparallel to one another, perpendicular or at least at the same angle tothe workpiece surface.

When cutting glass, it should be ensured in particular that thetransformation temperature T_(G) of the respective glass is notexceeded.

If a microcrack is made at the start of the cutting line, the crackfollows the cutting line very precisely.

It is preferable for the respective laser beams to be beams of one ormore CO or CO₂ lasers, the wavelength of the laser particularlypreferably being tunable, so that it can be matched to the correspondingabsorption maximum of the material which is to be cut. The moresuccessfully the laser wavelength can be matched to the absorptionmaximum of the material to be cut, the more effective the induction of athermomechanical stress and also the higher the cutting speed which canbe selected.

The fluid cooling medium is preferably blown onto the workpiece from thetop downward.

Preferred fluid cooling media are cooling gases, preferably cold air, ora water/air mixture.

In a preferred embodiment of the invention, the temperature of the fluidcooling medium is set and controlled. In this case, it is particularlypreferable for the fluid cooling medium to be additionally cooled. Thiscan be achieved, for example, by means of at least one Peltier element.This allows reproducible process management under conditions which canbe controlled and monitored.

In a further configuration of the invention, the gas jet is shaped byone or more concentric and/or elliptical nozzles.

It is possible to use all gas-jet shapes and nozzle arrangements whichallow effective cooling.

In principle, all predetermined laser-beam shapes and associatedintensity distributions can be applied to the workpiece which is to becut.

A U-shaped or V-shaped contour of the laser beam and the associatedintensity distribution, as described in EP 0 872 303 A2, is particularlypreferred.

Furthermore, the CO or CO₂ laser, like any other laser which issufficiently strongly absorbed by the material, is suitable for thesubsequent fusion and rounding of the edge which is broken off as asharp edge.

The method according to the invention is particularly suitable forcutting brittle materials comprising glass, glass-ceramic or ceramic; itis advantageously possible to cut even relatively thick materials, forexample flat glass with a thickness of 30 mm.

With regard to the device for the rapid cutting of a workpiece made frombrittle material by means of laser beams along a predetermined cuttingline of any desired contour, the following are provided:

one or more laser sources for generating laser beams,

optical means for the coupled guidance of the laser beams focussed ontothe cutting line, without melting the material,

means for shaping the respective laser beams in such a manner that thebeam cross section acting as a focal spot on the surface of theworkpiece to be cut corresponds to a predetermined shape and intensitydistribution,

at least one drive arrangement for generating a relative movementbetween the laser beams and the workpiece along the cutting line, with athermomechanical stress being induced, and

means for blowing on a fluid cooling medium for subsequent cooling ofthe heated cutting-line section, so that the thermomechanical stress isincreased to above the breaking strength of the material.

According to a first embodiment, the optical means for coupled and inparticular parallel guidance of the laser beams allow coupled guidanceof the laser beams onto the cutting line in focused form and one behindthe other. In this case, it is possible for the optical means for thecoupled guidance of the laser beams to allow coupled guidance of thelaser beams in such a manner that the laser beams are guided onto thecutting line in focused form and separately from one another or,alternatively, completely or partially superimposed.

Furthermore, it is preferable to provide means for forming a microcrackat the start of the cutting line.

The laser is preferably a CO₂ laser, the wavelength of which correspondsto the spectral absorption maximum of the material which is to be cut.This CO₂ laser emits light in the far infrared region at a wavelength of10.6 μm. This thermal radiation has significant particular features whenacting on material. For example, it is highly absorbed by most materialswhich are transparent in visible light.

The fact of high absorption in glass is used to cut glass. At anabsorption coefficient of 10³ cm⁻¹, 95% of the power is absorbed in a 30μm thick layer.

If materials whose absorption maxima lie at a wavelength ofapproximately 5 μm are to be cut, it is recommended to use a CO laser asthe source.

In principle, it is possible to use all predetermined means forgenerating, shaping and guiding a laser beam which are suitable forinducing a thermomechanical stress below the melting temperature of theworkpiece which is to be cut.

According to a further embodiment, the gas jet for blowing clear thatpart of the surface of the workpiece on which the laser beam impinges isan air jet.

BRIEF DESCRIPTION OF THE DRAWING

The objects, features and advantages of the invention will now beillustrated in more detail with the aid of the following description ofthe preferred embodiments, with reference to the accompanying figures inwhich:

FIG. 1 is a diagram showing a first embodiment of a method of rapidlycutting a brittle workpiece, for example made of glass, along apredetermined cutting line:

FIG. 2 is a diagram showing a second embodiment of a method of rapidlycutting a brittle workpiece, for example made of glass, along apredetermined cutting line; and

FIG. 3 is a flow chart showing the steps of the method for rapid cuttingof a workpiece made of brittle material according to the invention.

As shown in FIGS. 1 and 2, for rapid cutting of a workpiece made frombrittle material by means of laser beams along a predetermined cuttingline 31 of any desired contour (indicated by alternating dashes anddots), in each case three laser beams are generated (step 1, FIG. 3),which are focused with coupled guidance onto the cutting line, withoutthe material being melted (steps 2 and 3, FIG. 3). The laser beams arepreferably guided perpendicular to the surface of the material. Thelaser beams are shaped in such a manner that the respective beam crosssection, which acts as a focal spot 11 on the surface of the workpieceto be cut, corresponds to a predetermined shape and intensitydistribution (step 4, FIG. 3). According to FIG. 1. the respective beamcross section has a V-shaped contour and the associated intensitydistribution. In the embodiment shown in FIG. 2, the respective beamcross section acting as a focal spot 11′, by contrast, has a linearcontour.

By generating a relative movement between the three coupled laser beamsand the workpiece along the cutting line (in the direction of the arrow41), a thermomechanical stress is induced in the workpiece (step 5, FIG.3). It is essential for the parallel coupled laser beams to follow thepredetermined cutting line 31 as accurately as possible. Following thelaser beams, as a result of a fluid cooling medium being blown on forsubsequent cooling (cooling spot 21) of the heated cutting-line section,the thermomechanical stress is increased to above the breaking strengthof the material (step 6, FIG. 3).

The three parallel laser beams are guided simultaneously, preferably onebehind the other, onto the cutting line.

What is claimed is:
 1. A method for rapid cutting of a workpiece along apredetermined cutting line of any desired shape by means of laser beams,said workpiece being made of brittle material, said method comprisingthe steps of: a) generating the laser beams; b) focusing the laser beamsonto the cutting line to form focused laser beams on the cutting line;c) guiding the focused laser beams one behind the other along thecutting line without melting the brittle material; d) shaping therespective laser beams so that respective beam cross-sections formingcorresponding focal spots on a surface of the workpiece havepredetermined shapes and intensity distributions; e) moving theworkpiece and the laser beams relative to each other so that the focalspots move along the cutting line and the focused laser beams induce athermo-mechanical stress in the brittle material without melting thebrittle material; and f) guiding a cooling spot, which is produced byblowing a fluid cooling medium onto a heated cutting line section of theworkpiece, along the cutting line following said focal spots so thatsaid cooling spot increases said thermo-mechanical stress produced bythe focused laser beams in the brittle material above a breakingstrength of the brittle material.
 2. The method as defined in claim 1,wherein during the guiding of the laser beams along the cutting line thefocused laser beams are completely or partially superimposed onto thecutting line.
 3. The method as defined in claim 1, wherein a microcrackis formed at a starting part of the cutting line.
 4. The method asdefined in claim 1, wherein the generating of the laser beams isperformed by a CO or CO₂ laser.
 5. The method as defined in claim 1,wherein the generating of the laser beams is performed by tunablelasers.
 6. The method as defined in claim 1, wherein the blowing of thefluid cooling medium occurs from above down onto the workpiece.
 7. Themethod as defined in claim 1, wherein the fluid cooling medium is acooling gas.
 8. The method as defined in claim 7, wherein said coolinggas is cold air.
 9. The method as defined in claim 1, wherein the fluidcooling medium is a mixture of air and water.
 10. The method as definedin claim 1, further comprising setting and controlling a temperature ofthe fluid cooling medium.
 11. The method as defined in claim 1, furthercomprising cooling the fluid cooling medium.
 12. The method as definedin claim 11, wherein the cooling of the fluid cooling medium isperformed by means of a Peltier element.
 13. A device for rapid cuttingof a workpiece along a predetermined cutting line of any desired shapeby means of laser beams, said workpiece being made of brittle material,said device comprising: laser means for generating the laser beams, saidlaser means comprising at least one laser source; optical means forfocusing the laser beams onto the cutting line to form focused laserbeams on the cutting line; optical means for guiding the focused laserbeams one behind the other along the cutting line without melting thebrittle material; means for shaping the respective laser beams so thatrespective beam cross-sections forming corresponding focal spots on asurface of the workpiece have predetermined shapes and intensitydistributions; drive means for moving the workpiece and the laser beamsrelative to each other along the cutting line so that the focal spotsmove along the cutting line and the focused laser beams induce athermo-mechanical stress in the brittle material without melting thebrittle material; and means for blowing a fluid cooling medium onto aheated cutting line section of the workpiece to form a cooling spot; andmeans for guiding said cooling spot along said cutting line followingsaid focal spots so that said cooling spot increases saidthermo-mechanical stress produced by the focused laser beams in thebrittle material above a breaking strength of the brittle material. 14.The device as defined in claim 13, wherein the optical means for guidingof the focused laser beams along the cutting line guides the focusedlaser beams so that the focused laser beams are completely or partiallysuperimposed onto the cutting line.
 15. The device as defined in claim13, further comprising means for making a microcrack at a starting partof the cutting line.
 16. The device as defined in claim 13, wherein thelaser means for generating the laser beams comprises a CO laser or a CO₂laser.
 17. The device as defined in claim 13, wherein the laser meansfor generating the laser beams comprises at least one tunable laser. 18.The device as defined in claim 13, wherein said means for blowing thefluid cooling medium blows the cooling medium from above down onto theworkpiece.
 19. The device as defined in claim 13, wherein said means forblowing the fluid cooling medium comprises means for blowing a coolinggas onto the workpiece.
 20. The device as defined in claim 13, whereinsaid means for blowing the fluid cooling medium comprises means forblowing cold air onto the workpiece.
 21. The device as defined in claim13, wherein said means for blowing the fluid cooling medium comprisesmeans for blowing a mixture of air and water onto the workpiece.
 22. Thedevice as defined in claim 13, further comprising means for setting andcontrolling a temperature of the fluid cooling medium.
 23. The device asdefined in claim 13, further comprising means for cooling the fluidcooling medium.
 24. The device as defined in claim 23, wherein the meansfor cooling of the fluid cooling medium comprises a Peltier element.